ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 11, pp. 2373–2376. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © N.M. Karpyak, R.G. Makitra, I.P. Polyuzhin, G.A. Marshalok, Ya.P. Koval’skii, 2009, published in Zhurnal Obshchei Khimii, 2009,
Vol. 79, No. 11, pp. 1851–1854.
Substituents Influences on the Rate
of α-Alkylacroleins Dimerization
N. M. Karpyak, R. G. Makitra, I. P. Polyuzhin, G. A. Marshalok, and Ya. P. Koval’skii
“Lviv Polytechnology” National University, ul. S. Bandery, 12, Lviv, 79013 Ukraine
e-mail: natakarpyak@mail.ru
Received April 22, 2009
Abstract—Kinetics of α-alkylacroleins dimerization was studied. It was established that both electronic and
steric factors affect the process rate, with the prevalence of the latter factors.
DOI: 10.1134/S1070363209110152
Diels–Alder reactions are sensitive to the influence
of substituents in the molecules of diene and
dienophile due to inductive and steric effects that has
been the subject of some investigations [1, 2]. For
example, bulky substituents in dienophile or diene can
hinder interaction between components. Intramolecular
van der Waals repulsion is possible also between sub-
stituents in diene [2]. However, the investigation
results reported in the literature concern, as a rule, the
interaction between dienes and dienophiles containing
various substituents or cycles. This makes difficult the
estimation of the separate influence of inductive and
steric effects on the process kinetics.
Linear dependence between values log k and 1/T is
observed with a satisfactory accuracy (R 0.998), what
allows to determine activation energy E for acrolein
dimerization process at 32.7 kJ mol–1.
We determined the dimerization rate values for α-
methyl-, α-ethyl-, and α-butylacroleins in the tem-
perature range of 160–200°С to evaluate the influence
of alkyl substituents on the α-carbon atom on the
process kinetics (Table 1). These data were compared
with similar values of acrolein dimerization calculated
for the same temperatures using the activation energy
value and the values of constants at 110–150°С. For
these compounds dependences between log k and 1/T
are also satisfactorily linear. The activation energies
calculated for α-alkylacroleins dimerization process
are given in Table 2. Their values correspond to the
values of Arrhenius activation energy for the majority
of bimolecular Diels–Alder cyclization reactions [1].
In this work we studied kinetics of α-alkylacroleins
dimerization, which proceeded by the mechanism of
Diels–Alder condensation. Since the unsuturated alde-
hyde acts here simultaneously as diene and dienophile
alike, the influence of alkyl substituents on the reaction
kinetics allows us to estimate these two effects
separately.
Although alkyl substituents show certain inductive
effect on the reaction rate, sharp decrease in the rate
with the increasing substituent size suggests the
prevailing influence of steric factors. Therefore to
estimate separately the effect of these two factors we
plotted the dependences of log k on the parameters σ*
and ES (Figs. 1, 2).
Analysis of the dimerization kinetics of unsub-
stituted acrolein in the temperature range 110–150°С
shows that the process rate increases with the growing
temperature. The rate constants of its dimerization (k ±
Δk) × 105, l mol–1 s–1 are as follows:
However, as shown in Figs. 1, 2, in both cases the
linearity is not observed for all temperatures.
Therefore, the data from Table 1 were generalized by
us using Hammet–Taft equation, which makes possible
the allowance for the combined influence of inductive
and steric factors on the reaction rate.
110
120
130
140
150
Acrolein 1.97±0.09 2.71±0.10 3.35±0.12 4.31±0.18 5.24±0.17
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